Electrochromic materials have the ability to change color in response to an applied electric field. Electrochromic materials have been used to enhance the functionality of displays used on billboards, in automobiles, and for other purposes.
One type of electrochromic material, referred to as an electrophoretic material, is commonly used in e-reader devices. For example, a simple implementation of an electrophoretic display comprises an apparatus in which titanium dioxide particles approximately one micrometer in diameter are dispersed in a hydrocarbon oil. A dark colored dye is added to the oil, along with surfactants and charging agents that cause the particles to take on an electric charge. This mixture is placed between two parallel, conductive plates separated by a gap of 10 to 100 micrometers. When a voltage is applied across the two plates, the particles will migrate electrophoretically to the plate bearing the opposite charge from that on the particles. When the particles are located at the front (viewing) side of the display, it appears white, because light is scattered back to the viewer by the high-index titania particles. When the particles are located at the rear side of the display, it appears dark, because incident light is absorbed by the colored dye. In this manner, the color of a single pixel can be controlled to be alternatively white or dark.
More recently, electrochromic materials have been developed that can emit a wider range of colors, including red, green, blue, yellow, etc., when different voltages are applied. For example, a variety of technologies have been developed to enable control of the color of individual pixels on a display or surface, such as materials using different colored particles (such as red, green, blue), variously colored filters, subpixels, materials comprising photonic crystals, electrochromic polymer films, etc.
Processing devices, such as personal computers, laptop computers, tablet devices, and cell phones, use various methods to alert or otherwise inform users of certain events. For example, many personal computers, laptop devices, etc., display alert messages on a display screen when a malfunction occurs. Similarly, when battery power is low, many processing devices alert users by displaying a message on the display screen. Many cell phones, personal computers, laptop computers and tablet devices may display a small battery icon in a corner of the display screen with an indication of the amount of power remaining. Similarly, many cell phones and other processing devices display a series of bars in a corner of the screen to indicate the current level of “reception,” i.e., whether or not a connection to a communication network is currently available.